Why is the healer class basically a one spell dps class?

[Ayche]

What? This works brilliant on rdm.

Isn't that because RDM does not really have a rotation, but priority rules it follows? So it can stop to do a healer duty and not lose as much as something like BLM would lose if they were interrupted for let's say 5 seconds or more.
But I cannot really speak for both BLM or RDM, I have not played both to max level.
I guess Summoner too could find a moment between Bahamut and Phoenix to do healing duty, but it would be kinda awkward to pop either and then realize that I need to go heal someone because they got hit by raid boss. If I am an actual healer, Glare is a Glare is a Glare. Since they are all equal in contribution, I do not need to worry about missing out on an extra glare.